Studies are being conducted to identify and characterize different malarial antigens and their respective antibodies in order to determine their relative role in the overall immune response. Various host parasite models are used in these studies including Plasmodium knowlesi and P. cynomolgi in rhesus monkeys, P. falciparum in owl monkeys and P. berghei and p. vinckei in rodents. Antigenic variation in malaria is being studied in vivo and in vitro using the schizont agglutination test to monitor antigenic differences between parasite populations during acute and chronic malarial infections. The effect of host immunity on the invasion of erythrocytes by merozoites is being investigated in an attempt to correlate antimerozoite activity with functional immunity to malaria. We also are evaluating and comparing methods of preparing malarial antigens for the indirect hemagglutination test.